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<p>Allows a function to be called without providing one or more trailing arguments.
</p>
<p>Indicated by using the following syntax for a parameter in the <span class="t-spar">parameter-list</span> of a <a href="function.html" title="cpp/language/function">function declaration</a>.
</p>
<table class="t-sdsc-begin">

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<td colspan="10" class="t-sdsc-sep">
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<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">declarator</span> <code><b>= </b></code> <span class="t-spar">initializer</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td>
</tr>



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</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td>
</tr>



<tr>
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<p>Default arguments are used in place of the missing trailing arguments in a function call:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> point<span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">3</span>, <span class="kw4">int</span> y <span class="sy1">=</span> <span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span>
 
point<span class="br0">(</span><span class="nu0">1</span>,<span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls point(1,2)</span>
point<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// calls point(1,4)</span>
point<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// calls point(3,4)</span></pre></div></div>
<p>In a function declaration, after a parameter with a default argument, all subsequent parameters must
</p>
<ul>
<li> have a default argument supplied in this or a previous declaration; or
</li>
<li> <span class="t-rev-inl t-since-cxx11"><span>be a function parameter pack.</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>
</li>
</ul>
<p>The ellipsis is not a parameter, and so can follow a parameter with a default argument.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">1</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error, assuming there's no previous declaration of f</span>
<span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span>, ...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// ok</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="me1">T</span><span class="sy1">&gt;</span> <span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span>, T... <span class="me1">args</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// ok</span></pre></div></div>
<p>Default arguments are only allowed in the parameter lists of <a href="function.html" title="cpp/language/function">function declarations</a><span class="t-rev-inl t-since-cxx14"><span> and <a href="lambda.html" title="cpp/language/lambda">lambda-expressions</a>,</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span> and are not allowed in the declarations of pointers to functions, references to functions, or in <a href="typedef.html" title="cpp/language/typedef">typedef</a> declarations. Template parameter lists use similar syntax for their  <a href="template_parameters.html#Default_template_arguments" title="cpp/language/template parameters">default template arguments</a>.
</p>
<p>For non-template functions, default arguments can be added to a function that was already declared if the function is redeclared in the same scope. At the point of a function call, the defaults are a union of the defaults provided in all visible declarations for the function. A redeclaration cannot introduce a default for an argument for which a default is already visible (even if the value is the same). A re-declaration in an inner scope does not acquire the default arguments from outer scopes.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>     <span class="co1">// #1 </span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #2 OK: adds a default</span>
 
<span class="kw4">void</span> h<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    f<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #1 and #2 are in scope; makes a call to f(3,7)</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span><span class="nu0">1</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: inner scope declarations don't acquire defaults</span>
<span class="br0">}</span>
 
<span class="kw4">void</span> m<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// new scope begins</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// inner scope declaration; has no defaults.</span>
    f<span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: not enough arguments to call f(int, int)</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">6</span><span class="br0">)</span><span class="sy4">;</span> 
    f<span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: calls f(4,6);</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">6</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: cannot redeclare a default in the same scope</span>
<span class="br0">}</span>
 
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">1</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #3 OK, adds a default to #2</span>
 
<span class="kw4">void</span> n<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// new scope begins </span>
    f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #1, #2, and #3 are in scope: calls f(1, 7);</span>
<span class="br0">}</span></pre></div></div>
<p>If an <a href="inline.html" title="cpp/language/inline">inline</a> function is declared in different translation units, the accumulated sets of default arguments must be the same at the end of each translation unit.
</p>
<p>If a <a href="friend.html" title="cpp/language/friend">friend</a> declaration specifies a default, it must be a friend function definition, and no other declarations of this function are allowed in the translation unit.
</p>
<p>The <a href="namespace.html#Using-declarations" title="cpp/language/namespace">using-declaration</a> carries over the set of known default arguments, and if more arguments are added later to the function's namespace, those defaults are also visible anywhere the using-declaration is visible
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> N <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">using</span> N<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    f<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls f(7, 1);</span>
    f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// error</span>
<span class="br0">}</span>
<span class="kw1">namespace</span> N <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">2</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">void</span> h<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// calls f(2, 1);</span>
<span class="br0">}</span></pre></div></div>
<p>The names used in the default arguments are looked up, checked for <a href="access.html" title="cpp/language/access">accessibility</a>, and bound at the point of declaration, but are executed at the point of the function call:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> f<span class="br0">(</span>a<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// lookup for f finds ::f, lookup for a finds ::a</span>
                     <span class="co1">// the value of ::a, which is 1 at this point, is not used</span>
<span class="kw4">void</span> h<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
  a <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>  <span class="co1">// changes the value of ::a</span>
  <span class="br0">{</span>
     <span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span>
     g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// calls f(2), then calls g() with the result</span>
  <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>For a <a href="member_functions.html" title="cpp/language/member functions">member function</a> of a non-template class, the default arguments are allowed on the out-of-class definition, and are combined with the default arguments provided by the declaration inside the class body. If these out-of-class defaults would turn a member function into a default, copy, or move constructor the program is ill-formed. For member functions of class templates, all defaults must be provided in the initial declaration of the member function.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> C <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">int</span> i, <span class="kw4">int</span> j <span class="sy1">=</span> <span class="nu0">99</span><span class="br0">)</span><span class="sy4">;</span>
    C<span class="br0">(</span><span class="kw4">int</span> arg<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// non-default constructor</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">void</span> C<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">3</span><span class="br0">)</span> <span class="br0">{</span>         <span class="co1">// error: default argument already</span>
<span class="br0">}</span>                              <span class="co1">// specified in class scope</span>
<span class="kw4">void</span> C<span class="sy4">::</span><span class="me2">g</span><span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">88</span>, <span class="kw4">int</span> j<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// OK: in this translation unit,</span>
<span class="br0">}</span>                              <span class="co1">// C::g can be called with no argument</span>
C<span class="sy4">::</span><span class="me2">C</span><span class="br0">(</span><span class="kw4">int</span> arg <span class="sy1">=</span> <span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>   <span class="co1">// Error: turns this into a default constructor</span>
<span class="br0">}</span></pre></div></div>
<p>The overriders of <a href="virtual.html" title="cpp/language/virtual">virtual</a> functions do not acquire the default arguments from the base class declarations, and when the virtual function call is made, the default arguments are decided based on the static type of the object (note: this can be avoided with <a rel="nofollow" class="external text" href="http%3A//www.gotw.ca/publications/mill18.htm">non-virtual interface</a> pattern).
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> Base <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Derived <span class="sy4">:</span> Base <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> a<span class="br0">)</span> override<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">void</span> m<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    Derived d<span class="sy4">;</span>
    Base<span class="sy3">&amp;</span> b <span class="sy1">=</span> d<span class="sy4">;</span>
    b.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: calls Derived::f(7) </span>
    d.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: no default </span>
<span class="br0">}</span></pre></div></div>
<p>Local variables are not allowed in default arguments:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> 
<span class="br0">{</span>
    <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw4">extern</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: local variable cannot be a default</span>
<span class="br0">}</span></pre></div></div>
<p>The <a href="this.html" title="cpp/language/this">this</a> pointer is not allowed in default arguments:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> A <span class="br0">{</span>
  <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy2">*</span> p <span class="sy1">=</span> this<span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// error: this is not allowed</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Non-static class members are not allowed in default arguments (even if they are not evaluated), except when used to form a pointer-to-member or in a member access expression.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> b<span class="sy4">;</span>
<span class="kw1">class</span> X <span class="br0">{</span>
  <span class="kw4">int</span> a<span class="sy4">;</span>
  <span class="kw4">int</span> mem1<span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: non-static member cannot be used</span>
  <span class="kw4">int</span> mem2<span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: lookup finds X::b, the static member</span>
  <span class="kw4">static</span> <span class="kw4">int</span> b<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Function parameters are not allowed in default arguments (even if they are not evaluated). Note that parameters that appear earlier in the parameter list are in <a href="scope.html" title="cpp/language/scope">scope</a>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> a<span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b <span class="sy1">=</span> a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: the parameter a used in a default argument</span>
<span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b <span class="sy1">=</span> sizeof<span class="br0">(</span>a<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: parameter a is used in a default argument</span></pre></div></div>
<p>The default arguments are not part of the function type
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">void</span> h<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  <span class="kw4">int</span> j <span class="sy1">=</span> f<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
  <span class="kw4">int</span> k <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// calls f(0);</span>
<span class="br0">}</span>
<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>p1<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy1">=</span> <span class="sy3">&amp;</span>f<span class="sy4">;</span>
<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>p2<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span>    <span class="sy1">=</span> <span class="sy3">&amp;</span>f<span class="sy4">;</span> <span class="co1">//Error: the type of f is int(int)</span></pre></div></div>
<p><br>
If a <a href="lambda.html" title="cpp/language/lambda">lambda-expression</a> appears in a default argument, it cannot explicitly or implicitly capture anything.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f2<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw4">void</span> g1<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="br0">(</span><span class="br0">[</span>i<span class="br0">]</span><span class="br0">{</span> <span class="kw1">return</span> i<span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: captures something</span>
    <span class="kw4">void</span> g2<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="br0">(</span><span class="br0">[</span>i<span class="br0">]</span><span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: captures something</span>
    <span class="kw4">void</span> g3<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="br0">(</span><span class="br0">[</span><span class="sy1">=</span><span class="br0">]</span><span class="br0">{</span> <span class="kw1">return</span> i<span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: captures something</span>
    <span class="kw4">void</span> g4<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="br0">(</span><span class="br0">[</span><span class="sy1">=</span><span class="br0">]</span><span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// OK: capture-less</span>
    <span class="kw4">void</span> g5<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="br0">(</span><span class="br0">[</span><span class="br0">]</span><span class="br0">{</span> <span class="kw1">return</span> sizeof i<span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: capture-less</span>
<span class="br0">}</span></pre></div></div>

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